An unusual case of orf

Diseases don’t always behave exactly as we expect them too. Sometimes, even when we think we’ve worked out most things about them, they can surprise us.

A case study published in Dermatology Online Journal – Orf parapoxvirus infection from a cat scratch by J Frandsen et al (see below for full ref) – is a nice example of this.

A Bit of Background

Electronmicrograph of orf virus

Orf (AKA ‘contagious pustular dermatitis’ or ‘ecythema contagiosum’) is caused by a parapoxvirus infection. (Parapoxviruses are double-stranded DNA viruses and other diseases caused by these viruses include bovine papular stomatitis and pseudocowpox). The orf virus is found across the world and is carried by sheep and goats. It is also zoonotic.

In sheep and goats the virus can cause disease at any time but generally causes the most problems in and just after the lambing season. In infected animals it manifests as pustular, scabby lesions. These scabs are full of virus. A new animal becomes infected if it has damaged skin that comes into contact with the virus, usually via direct contact with an infected animal.

We can become infected in exactly the same way and it is a known risk during the lambing season. Luckily orf infection is generally self-limiting unless we/the animal are immunocompromised for some reason (e.g. already ill/taking immunosuppressant drugs etc.)

Sheep with orf scabs

However, transmission doesn’t always have to be via direct contact. Transmission of the virus by fomites has also been reported including infection after contact with sheep fences or burrs in the sheep’s wool.

What’s interesting about this case report is that the sheep host is much further removed from the human case.

The Case Report

The authors of the paper report the case of a woman who presented herself to her ‘primary care physician’ (I’m guessing that’s the equivalent to my GP unless anyone else can tell me any differently) with an ulcerating blister in a region where she had been scratched by a stray kitten 4 weeks earlier. She reported no contact with any sheep or goats and had not been in any fields where they were grazed but she did live near farmland with sheep on it. She was treated with antibiotics but the lesion just got worse.

After being referred to specialist dermatologists she had a skin biopsy. Under the microscope this biopsy was seen to have features that are characteristic of parapoxvirus infections – this included eosinophilic (pink staining) cytoplasmic (imagining a cell as a fried egg: in the egg white) inclusions for any budding histopathologists out there. There were other differential diagnoses that could also fit this pattern, including milkers nodules, but the authors state that:

“Because milker’s nodules are smaller and less likely to ulcerate than orf, we assume our patient had orf “

The authors do point out that they couldn’t identify the specific viral type, however, what they had learnt from the biopsy was enough to allow them to prescribe topical treatment for the woman and the lesion healed up in 10 weeks.

What this means…

Now we can’t be totally sure that this case was caused by the orf virus but if it was that means that the kitten had presumably been wandering around a sheep farm, picked up the virus on its claws (possibly from scab material on the ground?) then trotted away from the farm and towards where this woman found it. When it scratched the woman it effectively inoculated her with the virus and that is how it was able to get in.

I would suspect that (prior to this case report at least) if you were a medic presented with this case, when a woman reports absolutely no contact with sheep, sheep pasture etc. that would move orf quite far down your differential diagnosis list. This case reminds us that zoonotic diseases are not always transmitted the way we expect.